This invention pertains generally to rotary injection molding systems performing plastic parison preforms and more particularly to rotary injection molding systems for forming plastic parison preforms which are capable of suppressing flash.
In conventional non-rotating injection molding processes, a plastic parison, or preform is formed between a mandrel, or core pin and a molding cavity. The parison, or preform, is later blow molded into a finished article or reform. To form a mold, elements of the mold must be joined to form an enclosure. Assembly of the elements of the enclosure result in the formation of a mating interface which is the boundary between the surfaces of the elements of the enclosure which mate with one another to form the enclosed cavity.
In conventional non-rotating injection molding systems, the mating interface is formed by the mating surfaces of the mandrel and molding cavity, which abut with one another when they are joined to form the enclosure. The tolerances of the mating surfaces of the non-rotating injection molding systems can be made sufficiently precise, i.e., a gap space on the order of 0.0005 to 0.001 inches, to allow venting of mold gases while simultaneously suppressing the flow of polymer material (flash) in the mating interface. Gas venting and suppression of flash is achieved in non-rotating injection molding systems for injection pressures of polymer on the order of 20,000 psi. Since the mating surfaces of the molding cavity and mandrel are static, a seal can be provided by producing a clamping force which is greater than the force exerted by the plastic on the mating surfaces during the injection cycle. Mating surfaces are maintained in a sealed position until the plastic is solidified so as to prevent the flow of polymer onto the mating surfaces after disassembly.
However, in order to produce molded articles of greater strength, it is desirable to impart a preferred orientation to long chains of molecules in the polymer by rotating or oscillating the mandrel with respect to the mold cavity during the formation process, such as disclosed in U.S. Pat. No. 3,307,726, the disclosure of which is specifically incorporated herein by reference. While molding with rotation produces a superior molded article, complications arise in sealing a rotating mating interface to prevent the flow of flash material on the mating surfaces. Normal minimal tolerances which prevent the flow of flash material into the mating interface in a non-rotating objection molding machine are insufficient to suppress flash between rotating or oscillating mating surfaces due to shear heating of the polymer material by the moving surfaces which reduces polymer viscosity. Reduced polymer viscosity in the region of the mating interface causes the flow of flash material into the mating interface for mating surface tolerances normally utilized in non-rotating injection molding systems. Implementation of rotary molding machines in a high repetition automated rotary injection molding process has resulted in build-up of flash on the mating surfaces. As flash builds up on the mating surfaces, the tolerances of the mating interface go out of specification and the rotary molding machine must be shut down so that flash can be removed. Since clamp pressures imbed the flash material into the mating surface, scrapping is normally not effective to completely remove the flash material and flash material must consequently be removed using a solvent in a time consuming process of washing the mating surfaces. Flash problems have prevented implementation of the plastic rotary molding machine in a viable high speed, commercially valuable rotary molding process.
Attempts have been made to overcome the problems of flash deposited on the rotating mating interfaces, as disclosed in U.S. Pat. Nos. 3,371,387; 3,389,434; 3,500,503; and 4,083,568; the discloser of which is specifically incorporated herein by reference. In general, the above referenced patents attempt to overcome the problem of flash forming on rotating mating surfaces by providing a flash gap on the order of 0.0002 to 0.0005 inches, and a low pressure reservoir connected to the flash gap for containing flash material emitted by the flash gap. As shown in the above referenced patents, and especially U.S. Pat. No. 3,389,434, a flash gap is formed in the rotating interface between the mandrel end cavity mold directly adjacent the parison. Due to its precise dimensions, the flash gap functions as a seal which is capable of emitting flash into the low pressure reservoir during peak intervals which occur during the molding process. The reservoir operates to collect flash material produced between the rotating surfaces of the flash gap. Upon separation of the rotating mating surfaces, the flash particles are removed from the reservoir by the application of pressurized air. This process is intended to eliminate the need to shut down the machine to remove flash particles from the reservoir.
However, several problems exist in implementing the devices in the above referenced patents. For example, relatively high clamping pressures are still required on the rotating mating interface, even though a low pressure reservoir is utilized to prevent flash material from flowing into the mating interface. As a consequence of the high clamping pressures required between the rotating mating surfaces, the high tolerance required for the flash gap, i.e., 0.0002 to 0.0005 inches, are difficult to maintain due to the friction and consequent wear generated. Moreover, during the process of separation of the mandrel and molding cavity, flash particles removed from the reservoir by air pressure become airborne and deposit on the mating surfaces. Subsequent joining of the mating surfaces causes the previously airborne flash particles to embed in the mating interface requiring machine shutdown in the application of a solvent as disclosed above, to remove the embedded flash particles to maintain proper tolerances.